The present invention relates to a process for controlling the siding gloss in thermoplastic compositions comprising a blend of PVC resin and ASA graft copolymers.
Graft copolymers comprising a discountinous poly(alkyl(meth)acrylate rubber phase and a rigid thermoplastic phase, e.g., ASA graft copolymers, may be used in extrusion compositions for their desirable properties as impact modifiers. U.S. Pat. No. 6,054,531 discloses a method for controlling the swell index and gel content of an emulsion polymerized cross-linked polyacrylate rubber to improve its performance as an impact modifier in a thermoplastic material. EP Publication No. 913408 discloses a process for making a polyacrylate rubber in the presence of a surfactant having the formula Rxe2x80x94SO3M wherein R is an alkyl, and M is a hydrogen radical or a cation. Such a rubber is found to provide improved weatherability when used as an impact modifier in thermoplastic resin extrusion compositions.
Thermoplastic compositions comprising graft copolymers are often used in weatherable applications such as outdoor furniture or housing sidings. It is often desirable to control the gloss of the surface of such articles to improve aesthetics, e.g., for the sidings to have more of a wood-like appearance. One practice is to add abrasive fillers such as aluminum silicate or calcium carbonate to the thermoplastic compositions to give a matte surface. U.S. Pat. No. 4,945,131 teaches the regulation of the swell index of high impact polystyrene rubber phase to control the surface gloss of a blend of polyphenylene ether resin after molding. Going the reverse direction with respect to the effect of cross-linking on surface gloss, U.S. Pat. No. 5,130,374 discloses thermoplastic compositions with reduced gloss by the use of cross-linked PVC as an additive to conventional PVC siding and that a higher level of cross-linking will lead to lower gloss levels. U.S. Pat. No. 4,894,416 teaches the use of epoxide-containing resins to form additional cross-links as a means for reducing gloss levels in ASA resins.
Applicants have found that the gloss in blends comprising PVC and ASA graft copolymers can be controlled by the swell index in the polyacrylate rubber, preferably lowering the cross-linking index to within a controlled range.
The present invention relates to a method to control the surface gloss of a composition of: (a) a polyvinyl chloride resin and (b) a graft copolymer comprising a discontinuous poly(alkyl(meth)acrylate) rubber phase and a rigid thermoplastic phase, wherein at least a portion of rigid thermoplastic phase is chemically grafted to the poly(alkyl(meth)acrylate) rubber phase, by regulating the swell index of the rubber phase in the graft copolymer to at least about 8 to vary the surface gloss after extrusion or molding.
In one embodiment of the present invention, the swell index is regulated by controlling the amount of the polyethyleneically unsaturated monomer to about 0.10 to 0.40 parts by weight of the polyethyleneically unsaturated monomer per 100 parts by weight of the alkyl(meth)acrylate monomer in the emulsion polymerization reaction to prepare the poly(alkyl(meth)acrylate) rubber phase.
In a second embodiment of the present invention, the polyethyleneically unsaturated monomer which functions as the cross-linking and graft-linking monomer in the preparation of the poly(alkyl(meth)acrylate) rubber latex comprises triallyl cyanurate.